ISP Config and GoDaddy MX

I know this one has been done to death, but I still haven't grasped it. I'm not running a DNS server, single static IP and single site. I'm using the Ubuntu perfect, btw. I never quite figured out my email. I have my domain at godaddy, and I've changed the A host record, which has been working great, but I haven't played with mail until now. According to the autoresponder, godaddy still has control of the MX address. I don't know what to change it to, though. My IP address? Port 25 on the router is opened, too. Thanks everyone.